1. Overview
While inheritance enables us to reuse existing code, sometimes we do need to set limitations on extensibility for various reasons; the final keyword allows us to do exactly that.
In this tutorial, we’ll take a look at what the final keyword means for classes, methods, and variables.
2. Final Classes
Classes marked as final can’t be extended. If we look at the code of Java core libraries, we’ll find many final classes there. One example is the String class.
Consider the situation if we can extend the String class, override any of its methods, and substitute all the String instances with the instances of our specific String subclass.
The result of the operations over String objects will then become unpredictable. And given that the String class is used everywhere, it’s unacceptable. That’s why the String class is marked as final.
Any attempt to inherit from a final class will cause a compiler error. To demonstrate this, let’s create the final class Cat:
public final class Cat {
private int weight;
// standard getter and setter
}
And let’s try to extend it:
public class BlackCat extends Cat {
}
We’ll see the compiler error:
The type BlackCat cannot subclass the final class Cat
Note that the final keyword in a class declaration doesn’t mean that the objects of this class are immutable. We can change the fields of Cat object freely:
Cat cat = new Cat();
cat.setWeight(1);
assertEquals(1, cat.getWeight());
We just can’t extend it.
If we follow the rules of good design strictly, we should create and document a class carefully or declare it final for safety reasons. However, we should use caution when creating final classes.
Notice that making a class final means that no other programmer can improve it. Imagine that we’re using a class and don’t have the source code for it, and there’s a problem with one method.
If the class is final, we can’t extend it to override the method and fix the problem. In other words, we lose extensibility, one of the benefits of object-oriented programming.
3. Final Methods
Methods marked as final cannot be overridden. When we design a class and feel that a method shouldn’t be overridden, we can make this method final. We can also find many final methods in Java core libraries.
Sometimes we don’t need to prohibit a class extension entirely, but only prevent overriding of some methods. A good example of this is the Thread class. It’s legal to extend it and thus create a custom thread class. But its isAlive() methods is final.
This method checks if a thread is alive. It’s impossible to override the isAlive() method correctly for many reasons. One of them is that this method is native. Native code is implemented in another programming language and is often specific to the operating system and hardware it’s running on.
Let’s create a Dog class and make its sound() method final:
public class Dog {
public final void sound() {
// ...
}
}
Now let’s extend the Dog class and try to override its sound() method:
public class BlackDog extends Dog {
public void sound() {
}
}
We’ll see the compiler error:
- overrides
com.baeldung.finalkeyword.Dog.sound
- Cannot override the final method from Dog
sound() method is final and can’t be overridden
If some methods of our class are called by other methods, we should consider making the called methods final. Otherwise, overriding them can affect the work of callers and cause surprising results.
If our constructor calls other methods, we should generally declare these methods final for the above reason.
What’s the difference between making all methods of the class final and marking the class itself final? In the first case, we can extend the class and add new methods to it.
In the second case, we can’t do this.
4. Final Variables
Variables marked as final can’t be reassigned. Once a final variable is initialized, it can’t be altered.
4.1. Final Primitive Variables
Let’s declare a primitive final variable i, then assign 1 to it.
And let’s try to assign a value of 2 to it:
public void whenFinalVariableAssign_thenOnlyOnce() {
final int i = 1;
//...
i=2;
}
The compiler says:
The final local variable i may already have been assigned
4.2. Final Reference Variables
If we have a final reference variable, we can’t reassign it either. But this doesn’t mean that the object it refers to is immutable. We can change the properties of this object freely.
To demonstrate this, let’s declare the final reference variable cat and initialize it:
final Cat cat = new Cat();
If we try to reassign it we’ll see a compiler error:
The final local variable cat cannot be assigned. It must be blank and not using a compound assignment
But we can change the properties of Cat instance:
cat.setWeight(5);
assertEquals(5, cat.getWeight());
4.3. Final Fields
Final fields can be either constants or write-once fields. To distinguish them, we should ask a question — would we include this field if we were to serialize the object? If no, then it’s not part of the object, but a constant.
Note that according to naming conventions, class constants should be uppercase, with components separated by underscore (“_”) characters:
static final int MAX_WIDTH = 999;
Note that any final field must be initialized before the constructor completes.
For static final fields, this means that we can initialize them:
- upon declaration as shown in the above example
- in the static initializer block
For instance final fields, this means that we can initialize them:
- upon declaration
- in the instance initializer block
- in the constructor
Otherwise, the compiler will give us an error.
4.4. Final Parameters
The final keyword is also legal to put before method parameters. A final parameter can’t be changed inside a method:
public void methodWithFinalArguments(final int x) {
x=1;
}
The above assignment causes the compiler error:
The final local variable x cannot be assigned. It must be blank and not using a compound assignment
5. Conclusion
In this article, we learned what the final keyword means for classes, methods, and variables. Although we may not use the final keyword often in our internal code, it may be a good design solution.
As always, the complete code for this article can be found in the GitHub project.